1. Field of the Invention
This invention relates to a process for chemical vapor deposition of diamond films.
2. Discussion of the Background
Deposition of diamond films using CVD techniques has been well established. Numerous workers have used a plethora of techniques and source gases for diamond growth [see T. R. Anthony in Mat. Res. Soc. Symp. Proc. 162, 61 (1990) or see also P. K. Bachmann et al. in Diamond and Related Materials 1, 1 (1991)]. The techniques have included microwave-plasma assisted, hot filament assisted, dc plasma assisted, arc-jet discharges, rf plasma assisted, and oxy-acetylene-torch CVD techniques. The vast majority of the work depends on molecular hydrogen dissociation/activation in high-temperature plasma regions or in equivalent high temperature regions such as a hot filament or an oxy-acetylene torch. As a consequence of the sample temperature being much lower than the source temperature, there exists a superequilibrium of atomic hydrogen at the diamond growth surface. Thus, diamond growth proceeds once a sufficient amount of atomic hydrogen is produced. One role of the atomic hydrogen is to dissolve any graphite from the depositing diamond layer. Some of the earliest and simplest theories of diamond growth hypothesized that diamond CVD growth was a codeposition process involving the deposition of both graphite and diamond but in which the graphite was dissolved preferentially, resulting in stabilization of the diamond phase. Thus, providing an effective graphite etchant to dissolve graphite from a depositing diamond layer is critical in any diamond CVD process. Another role of the atomic hydrogen is to promote diamond formation through stabilization of the surface of diamond. Additional insight into the thermodynamics of diamond deposition has been provided by W. A. Yarbrough whose quasiequilibrium calculations have shown that, at high fractions of atomic hydrogen (greater than 0.1%), diamond condensation is preferred over graphite [see W. A. Yarbrough in Mat. Res. Soc. Symp. Proc. 192, 75 (1990)]. Hence, diamond deposition techniques need to generate a high fraction of atomic hydrogen to insure diamond promotion over graphite. In addition, with the deposition process involving carbon atom addition per unit time, the deposition process must also provide a critical absolute atomic hydrogen flux per unit time in order to stabilize the instantaneous growth surface. It is clear that, in practice, the high-temperature extremes of these physical and chemical sources are necessary in order to produce the atomic hydrogen concentrations necessary for diamond growth. Indeed, those sources which are the hottest (dc arc plasmas and oxy-acetylene torches) produce diamond at the fastest rates. This is a consequence of those sources having both high fractions of atomic hydrogen and high absolute concentrations.
Using heavy molecular hydrogen dilutions, a great many hydrocarbon, halocarbon, fluorocarbon, and organic sources have been used to produce diamond films. Typically, the promotion of diamond bonding over graphitic bonding is only accomplished when the percentage of hydrocarbon in the gas phase is small, insuring that the percentage of graphite deposited by the particular technique can be dissolved before graphitic phases can be incorporated into the carbon layers. Correspondingly, the best films are deposited with hydrocarbon percentages between 0.5-2.0%. Films deposited at higher concentrations show little if any evidence of diamond bonding from Raman analysis [see C. Hata and Y. Sato in New Diamond, 32-34 (1990)].
Atomic F, atomic O, molecular F.sub.2, and OH are also efficient in dissolving graphite. Indeed, variations from the traditional 95-99% H.sub.2 with 1-5% CH.sub.4 feed gasses with F, Cl, O, and OH additives have been accomplished. In addition, growth of diamond from oxy-acetylene flames has been accomplished using a 1:1.05 mix of O.sub.2 to C.sub.2 H.sub.2 as the premix entering the combustion flame [see L. M. Hansen et al. in Mater. Letters 7, 289 (1988)].
Recent published work by the assignee of the present invention has demonstrated that thermally-activated fluorine based processes have deposited diamond films [see R. A. Rudder, J. B. Posthill, and R. J. Markunas in Electronics Letters 25, 1220 (1989)]. This work (based on atomic fluorine in the process) dissolves graphite in an analogous manner to the atomic hydrogen in the conventional processes described above. Other workers at Rice University have used mixed hydrogen-halogen chemistries activated in a hot monel tube to deposit diamond particles on substrates removed from the hot zone [see D. E. Patterson et al. in Applications of Diamond Films and Related Materials, Elsevier Science Publications, 564-576, (1991)]. Those workers have used F.sub.2 /CH.sub.4 and H.sub.2 /CF.sub.4 gasses thermally activated in the hot zone to deposit diamond in cooler regions of the furnace. The activation results in highly exothermic reactions via the formation of HF. The chemical conversion at atmospheric pressure produces a hot chemical flame source wherein active fluorine and hydrogen can participate in diamond growth.
In related published work by the present assignee, it has been shown that halogens (for example CF.sub.4) can be added to a hydrogen-based plasma process to enhance diamond nucleation. This process deposits diamond films at much higher concentrations of CF.sub.4 than could be used if CH.sub.4 were used instead of CF.sub.4. Hence, fluorine liberated by the plasma plays a key role in promoting diamond growth.
O and OH chemistries have been exploited in plasma-assisted techniques. Small quantities of oxygen and water vapor have been added to microwave plasma reactors. It has been found that the small percentages (0.5-2%) of oxygen and the small percentage of water vapor (0-6%) improve the Raman spectra and decrease the temperature at which diamond can deposit [see Y. Saito et al. in J. Mat. Sci. 23, 842 (1988) or Y. Saito in J. Mat. Sci. 25, 1246 (1990)]. Higher percentages have been observed to degrade the diamond quality. Thus, microwave CVD reactor work has discouraged many researchers from pursuing discharges richer in oxygen or discharges containing high concentrations of OH.
In contrast to this work, Buck et al. have used pure methanol or Ar/methanol microwave discharges to deposit diamond at a pressure of 23 Torr [see M. Buck et al. in Mat. Res. Soc. Symp. Proc. 162, 97 (1990)]. This work demonstrates that diamond growth from microwave discharges is possible at higher O/C and O/H ratios of feed gas material. The addition of an inert gas to the microwave acts as a diluent and provides no means to chemically alter the O/C/H ratio, and hence this process provides no means to adjust the growth chemistry.
Yet another way to deposit diamond utilizing high O/C ratios is an oxy-acetylene flame. In an oxy-acetylene flame, oxygen (O.sub.2) and acetylene (C.sub.2 H.sub.2) are spontaneously reacted in a chemical flame. Similar to the microwave plasma diamond growth, the chemical flame is at an extremely high temperature 3000.degree. C. At those temperatures, the reactants (O.sub.2 and C.sub.2 H.sub.2) and the burn products (CO, CO.sub.2, H.sub.2 O) are in a partially dissociated state such that atomic H is readily available to the diamond growth surface.
The production of atomic hydrogen from the various plasma and chemical techniques involving highly energetic physical sources (dc plasmas, arc-discharges, microwave plasma, rf-plasmas, hot-filaments) and involving hot chemical sources (oxy-acetylene torches, atmospheric F.sub.2 /CH.sub.2 or H.sub.2 /CF.sub.4 reactors) has been necessary to maintain a critical atomic hydrogen concentration sufficient to produce diamond growth. Insufficient supply of atomic hydrogen to the diamond growth surface results in poor diamond growth or no diamond growth. Certainly, the theoretical work of W. A. Yarbrough adds some insight into the nature of the diamond deposition process. A critical fraction of atomic hydrogen is necessary in the gas phase in order for diamond to precipitate rather than graphite. Unfortunately, the field of diamond CVD is not advanced enough to quantitatively define the absolute concentration necessary for every growth pressure and temperature.
The necessity for these high-temperature physical and chemical sources is required to maintain a high, steady-state atomic hydrogen population which is in turn related to the lifetime of the atomic hydrogen species. Atomic hydrogen is extremely reactive and possesses the highest thermal velocity of any element. Once created, atomic hydrogen rapidly diffuses from the point of creation and reacts frequently to either recombine with itself through 3-body interactions or to form a hydride compound with another element. Examples of such losses are H--H recombination on CVD reactor walls and H--graphite interactions to form CH.sub.4. Both loss mechanisms (recombination and hydride formation) deplete the gas phase of atomic hydrogen. Because loss rates are high with the traditional molecular H.sub.2 -based process, the generation rate must compensate the loss in order to maintain a sufficient concentration of atomic hydrogen. This requirement for a high generation rate to offset the loss rate imposes the necessity for high-temperature physical or chemical sources in traditional diamond CVD. If the loss rate could be reduced, then low-temperature physical and chemical sources could be used. Alternatively, if the loss rate could be reduced, then the traditional high-temperature sources would generate even higher fluxes of atomic H, and consequently higher diamond deposition rates would be achieved.
The physical chemistry of diamond synthesis has focused on reactions carried out under these high-temperature, extremely energetic conditions. This is true for equilibrium approaches such as high pressure, high temperature synthesis from metal melts, and is also true for kinetic approaches in which kinetic constraints are used to select a product state (diamond) other than the equilibrium form (graphite). In short, many techniques each supported by different sorts of facilities have been developed to implement molecular hydrogen-based process chemistries based on the production of atomic hydrogen from highly energetic sources.